Generally, a carbine has the carbine buttstock, a round barrel, a forend with a band around it, and a barrel band around the magazine end and barrel. Some have saddle rings. Standard original '73's were 20" but could be ordered however. Current Uberti repros are 16" and 19".
A rifle has the crescent buttplate, and might have a pistol grip. The forend has a cap on the end. The barrels are octagonal or half-oct. and the magazine attaches to it by a hanger. Front sights are dovetailed. Original standard was 24" and many were round, but again anything could be ordered. Ubertis are 18", 20", 24", 30"
There are certainly exceptions. I've seen several pics of old '66 carbines that had the rifle buttstock.